Passengers leaving London's Heathrow today on flights bound for the U.S. were told that there would be no in-flight entertainment. They also faced rigorous extra checks of carry-on bags, as well as some body searches, before boarding. This comes on the heels of the attempted terrorist attack on Northwest Flight 253 from Amsterdam to Detroit on Christmas Day.

The suspension of in-flight entertainment is highly unusual. While officials remained tight-lipped about the steps they are taking and the reasons behind them, this particular decision may well reflect one of the most arresting details of yesterday’s incident—its timing.

Where, exactly, is the last line of defense against the increasingly inventive bomb designers?

All previous plans to destroy trans-oceanic airliners, most notably the foiled attempt, planned by mastermind Ramzi Yousef, in the mid-1990s to bring down 12 flights over the Pacific, and shoe-bomber Richard Reid’s attempt in 2001, have involved detonating bombs in mid-flight, which would make it extremely difficult to find the wrecks at great depths. In the case of Flight 253, the alleged terrorist waited until the airplane was on its approach to Detroit. The calculation might well have been to achieve a two-for-one: Destroy the airplane and wreak terror on the ground, too—as in the case of the Pan Am flight destroyed over Lockerbie, Scotland, in December 1988, in which many on the ground died. (In this case, the bomb had an altitude-sensitive trigger that was supposed to have worked over water, but the flight path differed from the expected one.)

What would aid an attack with this motive? The moving map found on the back of plane chairs. These maps are now so sophisticated that they accurately pinpoint not only the time left to landing but the exact flight path. That is like giving a Satnav device to the bomber.

The details of yesterday’s attack are still very speculative. By all accounts, the bomber was employing an incendiary device, not an explosive, probably because its components would have been a lot harder to detect. He was, it appears, overpowered as the contraption failed to properly ignite. At the core of all of this lies an issue that airport security officials have been forced to revisit once again: Where, exactly, is the last line of defense against the increasingly inventive bomb designers? The answer is, simply, before the bomber gets on the airplane. But if he does, what then? Yesterday it proved to be passengers and flight crew—involving acts of instinctive bravery and coolness. And now flight crews will need to be even more vigilant. It is also likely that there will be a renewed deployment of air marshals, traveling incognito, to bring another level of responder to international flights judged to be likely targets.